Q: I have
Nudy and Munirah. Nudy was breastfed by her mother's co-wife, Munirah's step-mother that is. Munirah was breastfed along with her half-brother Muhammad. It is worth mentioning that all the family members know that she is Muhammad's milk-sister based upon the rulings of breastfeeding. Is it permissible for me to keep both Nudy and Munirah in wedlock at the same time?
If the case is as you have mentioned, it will be impermissible to keep the above-mentioned two women as wives at the same time, as they are milk sisters. Allah (Glorified be He) states,
Forbidden to you (for marriage) are: your mothers
to His statement,
...and two sisters in wedlock at the same time, except for what has already passed
Moreover, the Prophet (peace be upon him) stated,
Relations through breastfeeding makes all those things unlawful which are unlawful through corresponding birth (blood) relations (i.e., prohibited marriages).
The questioner must therefore divorce the second wife. (Part No. 18; Page No. 233) May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!